Intensity-modulated radiation therapy dose maps: the matchline effect

In 2002 when this research started the brief of the project was to produce streamlined checks of planar dose maps delivered by IMRT fields to film. At this time no other centre in Australia had a protocol for checking accuracy of RTP planned RT dose distributions. While many US centers have been checking IMRT distributions, there is still no standard protocol for these checks. By the end of this project in 2005, 13 IMRT patient treatments had been successfully checked and this centre remains the only centre to have treated IMRT patients in Australia using the pinnacle RTP planning computer platform. Early film dose maps revealed dose spikes due to MLC matchline effects. These matchlines were due to Varian MLC leaf ends sometimes matching other segment neighbors and were not predicted using pinnacle RTP until version 7.4 available about 2 months prior to the end of this project cycle. Verifying a radiation treatment planning (RTP) computer\u27s IMRT calculation was the first task for this thesis. Planar dose maps (dose in water perpendicular to the beam [cGy/MU]) were compared with beam dose distributions measured using films (XV and EDR) at various depths. The RTP computer and film measurements agreed within &#;3% within the inside field region. In addition, the XV film had a lower linear dose response range than the EDR film, the efficacy of each film type depends on dose range, the XV being used predominantly for planar dose maps and EDR for combined axial dose maps. High dose lines (matchline effect) were studied with film measurement. Matchlines were caused by a contribution of extra penumbral dose from MLC transmission due to curved leaf ends. An MLC bank leaf stepping program was used with various minor overlap values (0, 0.06, 0.1, 0.14, 0.2 cm) of MLC position. With confirmation by BEAMnrc Monte Carlo simulations, a dosimetric overlap value due to collective effect of scatter and the rounded leaf end transmission equivalent to 0.09 cm leaf overlap was found for a particular weighting of each segment. Note the physical offset value set to avoid leaf collision is an additional 0.05 cm. An overlapping co-incident field technique was used to extend field size, this also showed a small jaw-leaf matchline effect at both edges of an overlap region. An aSi-EPID combined with Varian dosimetry software also showed matchline resolution similar to film. The aSi-EPID, XV film, Pinnacle RTP (version 7.0g and 7.4) and BEAMnrc Monte Carlo were all compared for a 25 segment step and shoot IMRT distribution. IMRT doses in the axial plane were further verified with an I�mRT phantom (Scanditronix-Wellhofer) using the EDR and a new low dose radiochromic film (Gafchromic� EBT, Lot no. 34267-004). For the irradiated perpendicular calibration setup, dose agreed to within &#;5% (1 SD) for EDR and \u274% (1 SD) for Gafchromic\u27 EBT film with RTP and an ionization chamber. The conclusions based on this thesis are the following; The matchlines represented a potential overdose to some small volumes within the target dose delivery. The matchline patterns produced by moving leaf banks in known sequences helped reveal the physics properties of the rounded leaf end. Appropriate physical leaf gaps were found to mask the matchline, however due to differences in segment weights these were not recommended. A Monte Carlo model of the Varian 120 MLC was developed using Beam NRC and this model predicted matchline effects. EPID dosimetry revealed an a-Si detector array had sufficient spacial resolution to show matchlines. Late in cycle Version 7.4 of RTP computer leaf model did predict matchlines of smaller magnitude than experimental results

Multi-institutional evaluation using the end-to-end test for implementation of dynamic techniques of radiation therapy in Thailand

AimIn this study, an accuracy survey of intensity-modulated radiation therapy (IMRT) and volumetric arc radiation therapy (VMAT) implementation in radiotherapy centers in Thailand was conducted.BackgroundIt is well recognized that there is a need for radiotherapy centers to evaluate the accuracy levels of their current practices, and use the related information to identify opportunities for future development.Materials and methodsAn end-to-end test using a CIRS thorax phantom was carried out at 8 participating centers. Based on each center's protocol for simulation and planning, linac-based IMRT or VMAT plans were generated following the IAEA (CRP E24017) guidelines. Point doses in the region of PTVs and OARs were obtained from 5 ionization chamber readings and the dose distribution from the radiochromic films. The global gamma indices of the measurement doses and the treatment planning system calculation doses were compared.ResultsThe large majority of the RT centers (6/8) fulfilled the dosimetric goals, with the measured and calculated doses at the specification points agreeing within ±3% for PTV and ±5% for OARS. At 2 centers, TPS underestimated the lung doses by about 6% and spinal cord doses by 8%. The mean percentage gamma pass rates for the 8 centers were 98.29±0.67% (for the 3%/3mm criterion) and 96.72±0.84% (for the 2%/2mm criterion).ConclusionsThe 8 participating RT centers achieved a satisfactory quality level of IMRT/VMAT clinical implementation